The combination of the spectral efficient PAM4 modulation together with the mature direct modulation / direct detection technology has been chosen by the #802.3bs and #802.3cd task forces [i] to address the urgent demand for more capacity and accelerating cost-per-bit reduction in intra and inter data center communication links. The shift from NRZ to #PAM4 modulation enables to double the line rates compared to optical #100Gigabit Ethernet transceivers while keeping the modulation speed at 26.56125Gbaud [ii] and therefore to leverage some of existing #100G components.
Consequently, the compliance test procedures considered for next-generation #400GBASE transceivers are similar to the ones adopted for NRZ-based #100GBASE transceiver but with following noticeable differences: A new #TDECQ metric[iii] is employed to characterize the quality of a transmitted/received signal instead of the classical eye mask analysis. In addition, a digital reference equalizer is required to compute various signal metrics during transmitter performance testing or during stress signal calibration for receiver stress testing. Finally, the optical transceiver is not expected to operate error-free any more - neither under the stress conditions defined by the standards nor in typical usage.
The latter point is a consequence of the significant sensitivity penalty (>9dB) resulting from the shift from NRZ to PAM4. This aspect affects the transceiver design, performance test and compliance in different ways: A Reed-Solomon forward error correction (#FEC) scheme is required. It is often located on the line-card and consequently not considered in the transceiver test procedures. Furthermore, over-dimensioning the transceiver design to ensure enough margin is not possible any more as the requirements for the components (direct modulated laser, integrated MZM, photodiode, driver and trans-impedance amplifier) in terms of noise, dynamic range and bandwidth are already very stringent. Careful design is therefore necessary to achieve the target performance using existing #100G optical components. Finally, accurate, stable and repeatable stress signal calibration () is required to ensure a reliable transceiver qualification process.
Unstressed (left) and stressed (right) 26Gbaud PAM4 signal according to the 400GBASE-FR8 recommendations (status of June 2017).
Achieving this is not a trivial task as the combination of different stress factors (inter symbol interference, jitter,
Gaussian noise, optical power level) has complex dependencies on the target metrics (figure below).
Keysight solutions for 400G optical transceiver testing address all these challenges. For instance, Keysight will showcase a fully automatized stress signal calibration and DUT compliance performance test for PAM4-based #transceiver testing at ECOC in Goteborg. This solution, based on the high performance BERT M8046A #64Gbaud and DCA-M scope, addresses the needs of R&D and QA engineers working on the next generation of optical transceivers.
For more information:
[i] “802.3bs 200 Gb/s and 400 Gb/s Ethernet Task Force” and “802.3cd, 50 Gb/s, 100 Gb/s and 200 Gb/s Ethernet Task Force”
[ii] Some flavors of the a.m. standard drafts also consider a doubling of the symbol rate to 53.123GBaud, resulting in a line rate of 106Gb/s.
[iii] King, Jonathan, David Leyba, and Greg LeCheminant. "TDECQ (transmitter dispersion eye closure quaternary) replaces historic eye-mask and TDP test for 400 Gb/s PAM4 optical transmitters." Optical Fiber Communication Conference. Optical Society of America, 2017.